Don't check privsep user or path when unprivileged

If running with privsep (mandatory now) as a non-privileged user, we
don't chroot or change to an unprivileged user however we still checked
the existence of the user and directory.  Don't do those checks if we're
not going to use them.  Based in part on a patch from Lionel Fourquaux
via Corinna Vinschen, ok djm@
diff --git a/sshd.c b/sshd.c
index 010a2c3..197c4ec 100644
--- a/sshd.c
+++ b/sshd.c
@@ -223,6 +223,7 @@
 int use_privsep = -1;
 struct monitor *pmonitor = NULL;
 int privsep_is_preauth = 1;
+static int privsep_chroot = 1;
 
 /* global authentication context */
 Authctxt *the_authctxt = NULL;
@@ -541,7 +542,7 @@
 	demote_sensitive_data();
 
 	/* Demote the child */
-	if (getuid() == 0 || geteuid() == 0) {
+	if (privsep_chroot) {
 		/* Change our root directory */
 		if (chroot(_PATH_PRIVSEP_CHROOT_DIR) == -1)
 			fatal("chroot(\"%s\"): %s", _PATH_PRIVSEP_CHROOT_DIR,
@@ -1640,8 +1641,9 @@
 	);
 
 	/* Store privilege separation user for later use if required. */
+	privsep_chroot = use_privsep && (getuid() == 0 || geteuid() == 0);
 	if ((privsep_pw = getpwnam(SSH_PRIVSEP_USER)) == NULL) {
-		if (use_privsep || options.kerberos_authentication)
+		if (privsep_chroot || options.kerberos_authentication)
 			fatal("Privilege separation user %s does not exist",
 			    SSH_PRIVSEP_USER);
 	} else {
@@ -1767,7 +1769,7 @@
 		    key_type(key));
 	}
 
-	if (use_privsep) {
+	if (privsep_chroot) {
 		struct stat st;
 
 		if ((stat(_PATH_PRIVSEP_CHROOT_DIR, &st) == -1) ||